Props to my friend David Hatfield for walking me through this...
In Financial, Open the Advanced Financial Reports window.
Duplicate the report, rename it. Be sure that 'profit and loss' is selected as report type. It should be, but the first time through 'balance sheet' was selected and I had to do it twice.
Open the layout
There are two keys. First, click on the 'C3' column heading and open the Financial Column Definition window. Edit it as shown, use the appropriate year. I'm doing this in January of 2021, so I'm looking back at a historical year.
Same with the C2 column
The second key is this. I needed to point the system clock back to 12/31/2020. Doesn't seem intuitive, but a developer, not a bean counter.
All set. You're welcome. Leave a comment?